1. Field of the invention
This invention relates to a process for producing lactic acid polymers useful as biodegradable plastics. More particularly, the invention relates to a process for rapid and continuous production of a high molecular weight lactic acid polymer and a shaped article thereof, which process comprises the steps of preparing a lactic acid prepolymer by the polycondensation of inexpensive lactic acid, feeding the prepolymer into a screw-type extruder and allowing only the by-product water to be distilled off to emerge from the reaction system, with lactide and other low molecular weight compounds being refluxed as they arise in the course of reaction.
Biodegradable plastics, which will degrade environmentally after having served their purposes to be eventually recycled to nature in the form of low molecular weight compounds, are one of the subjects of intensive studies being currently made. While many plastics are known to be biodegradable, aliphatic polyesters degrade completely to monomers under environmental stresses such as microorganisms and water so that they are eventually incorporated into natural material circulation in the form of carbon dioxide and water. To take advantage of this unique feature, researchers have recently started to review the possibility of expanding the use of aliphatic polyesters from the medical area to general-purpose industrial materials which are intended to be discarded into the environment after use.
Lactic acid polymers are most typical of the biodegradable plastics based on aliphatic polyesters since they are not only high in degradability and transparency but also so highly compatible with other polymers that they can be easily modified in characteristics. Additionally, they will readily degrade to monomers upon heating or addition of specific solvents. Hence, there exists a great demand for developing new uses of lactic acid polymers as materials that can be recycled in the monomeric form.
2. Related art
Lactic acid polymers are currently produced by two methods: in one method, lactide which is a cyclic dimer of lactic acid is used as a starting material and subjected to ring-opening polymerization; the other method comprises subjecting lactic acid to dehydrative polycondensation. From an engineering viewpoint, the first approach which starts with lactide is more advantageous since ionic polymerization causes a chain reaction to occur and because the product polymer has a very high molecular weight on the order of 10.sup.5. This process has already been established in the laboratory. See, for example, WO 90/01521, which teaches a process for producing polymers by the ring-opening polymerization of lactide, in which polymers having various characteristics are synthesized by either blending an optically active D- or L-lactide as a plasticizer with polymer or changing the mixing or copolymerization ratio of the D- or L-lactide to optically inactive meso D,L-lactide.
However, in order to expand the use of those polymers to the medical and packaging areas, they must have weight-average molecular weights of at least about 10.sup.5. When the polymers are to be shaped into films and other products, molecular weights of that order are necessary so that the shaped article will exhibit mechanical strength beyond a certain level while maintaining the initial (as-produced) quality throughout the use period without readily undergoing hydrolysis. In addition, the lactide must not contain impurities such as water and solvents that will retard the polymerization reaction. To meet this requirement, the lactide is purified by several runs of recrystallization with a solvent such as ethyl acetate and it has been proposed that lactide loss due to the sequence of purifying steps be minimized by a process in which the unpurified lactide is dissolved in a water-immiscible organic solvent and wherein the solution is subjected to extraction with a basic substance in aqueous solution, with the extracted portion being separated and processed to have an increased molecular weight.
JPA 88/165430 teaches a method comprising the steps of dissolving unpurified lactide in dichloromethane or ethyl acetate, subjecting the solution to extraction with an inert salt (e.g., sodium chloride or sodium sulfate) in aqueous phase, separating lactide from within the organic solvent, and homopolymerizing or copolymerizing the separated lactide.
Despite these complicated procedures, the purified lactide is so much hygroscopic and deliquescent that there are many precautions to be observed during handling, such as the need for storage in a diphosphorus pentasulfide atmosphere to insure complete removal of water. What is more, many disadvantages have been known from an engineering viewpoint, as typified by the need to replace the atmosphere in the polymerization reactor for ring-opening polymerization by a highly pure inert gas so that the effects of oxygen and water are eliminated as much as possible.
With a view to solving these problems, JPB 90/52930 has proposed a process in which lactic acid is heated and polycondensed in an inert gas atmosphere in the presence of a catalyst (at 180.degree. C. for 4 h in the disclosed examples) until the polycondensation reaction is completed at temperature within 220.degree.-260.degree. C. below vacuum level 10 mmHg (at 260.degree. C..times.2 mmHg for 8 h in the disclosed examples), whereby a lactic acid polymer having a molecular weight of 4,000-20,000 is directly produced from lactic acid.
This method is capable of converting lactic acid to its polymeric form in one step without passing through lactide, so it offers the advantage of significantly reducing the cost of polymer production. On the other hand, the method has the following problems (1)-(5) and no industrially feasible process has been established.
(1) The polycondensation of lactic acid involves a reaction system in which low molecular weight compounds that form as by-products in the reaction process such as dimers having a cyclic chain structure (e.g., lactide), lactic acid ethers and water maintain complex equilibria with the product polymer; hence, depending on the reaction control, side reactions may proceed preferentially or, if an equilibrium is reached between each of the by-products and the product polymer, the reaction for polymer production may stop at the point of time when such equilibrium is attained.
(2) The polycondensation of lactic acid is a reaction to formate ester bond between monomers; however, depending on the reaction conditions (temperature and the degree of vacuum), tin-base catalysts which are commonly used to accelerate the reaction may decompose the formed ester bonds to accelerate depolymerization reaction; as a result, the reaction for forming cyclic structures such as lactide predominates and the rate of the reaction for polymer formation may decrease.
(3) The dehydrative polycondensation of lactic acid can be further accelerated by allowing leaving components such as free water and by-product water to be distilled off from the system under vacuum but to this end, the degree of vacuum in the batch-type polymerization reactor has to be enhanced significantly; in this case, however, lactic acid oligomers and monomeric lactic acid in the reaction solution are prone to experience a phenomenon that may well be described as "bumping" and they will be distilled off together with the by-product water to cause occasional significant drops in the yield of the product polymer.
(4) When a conventional batch-type polymerization reactor is used for lactic acid polymer synthesis, the viscosity of the reacting solution increases with the progress of reaction and the resulting deficiency in the force of agitation and other problems will deteriorate the polymer surface refreshing action, causing the by-product water to be confined within the polymer in such a state that it is no longer easily removable.
(5) The reaction for converting lactic acid to a high polymer form by dehydrative polycondensation involves the elimination of water under heated conditions; however, due, not only to the low thermal stability of the product polymer, but also to the strong likelihood for hydrolysis to occur in the presence of the water that has been brought into the reaction system from the starting material or the by-product water, there is strong likelihood that the molecular weight of the product polymer will drop in the process of reaction.
In a method for making a final product, the lactic acid polymer is conventionally pelletized and the pellets are processed into a film, a container or other shapes. JPA 94/299054 proposes a method in which the lactic acid polymer is first crystallized and then injected or blow molded with care being taken to insure pellets will not thermally fuse together in the hopper of the molding machine.
However, the lactic acid polymer which has only low thermal stability will deteriorate due to the thermal history created in the pelletizing process and the quality of the shaped article is often affected in an undesirable manner. Additionally, if the lactic acid polymer is stored in the form of pellets as a molding material, hydrolysis due to moisture absorption may potentially lower the molecular weight of the polymer and, what is more, the residual water in the pellets can be a cause of deterioration in the physical properties of the shaped article being molded. To avoid these possibilities, it has been necessary to dry the pellets before they are molded. The addition of such an extra step unavoidably increases the operating cost of the conventional process, which therefore is not an industrially advantageous approach.